Thermionic converter with additives

ABSTRACT

A method and device for providing both electronegative and electropositive adsorbates to the emitter and collector surfaces of thermionic converters. The adsorbates are contained in a single reservoir and are provided at a temperature which is compatible with that at which the converter operates.

United States Patent Rufeh et al.

THERMIONIC CONVERTER WITH ADDITIVES Inventors: Firooz Ruteh; David P. Lieb, both of Lexington; Robert C. Howard, Wayland, all of Mass.

Assignee: Thermo Electron Corporation, Waltham,

Mass.

Filed: Oct. 24,1968

App]. No.: 770,325

u.s.c| ..310/4 rm. Cl. ..l-l0lj45/00 Field of Search ..313/174, 220-224;

[151 3,663,840 51 May 16, 1972 References Cited UNITED STATES PATENTS 11/1964 Haring et al. ..3 10/4 3/1965 Gunther et al 310/4 8/ l 965 Coleman ...3 l0/4 ll/l967 Jester et al ...3l0/4 4/1969 Langpape et a]. ..3 l0/4 2/1963 Opferman ..3l0/4 X Primary Examiner-D. F Duggan Attorney-Kenway, Jenney & Hildreth ABSTRACT A method and device for providing both electronegative and electropositive adsorbates to the emitter and collector surfaces of thennionic converters. The adsorbates are contained in a single reservoir and are provided at a temperature which is compatible with that at which the converter operates.

4 Claims, 1 Drawing Figure Patented May 16, 1972 INVENTORS FIROOZ UFEH DAVID LIEB ROBERT C. HOWARD Way/mg ATTORNEYS THERMIONIC CONVERTER WITH ADDITIVES The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457).

This invention relates generally to thermionic converters such as those described in U. S. Pat. No. 3,l29,345, of Hatsopoulos, entitled Process and Apparatus for Converting Thermal Energy Into Electrical Energy and assigned to the same assignee as the present invention.

A thermionic converter, which is sometimes called a thermo-electron engine, directly converts heat into electricity without utilizing moving parts. The converter includes an emitter which serves as a cathode, and a collector closely spaced from the emitter serving as an anode, both elements being enclosed in a vacuum envelope. The emitter cathode is heated to an extremely high temperature of the order of 2,000 K., while the collector-anode is maintained at a much lower temperature on the order of 1,000 K. Electrons are driven from the emitter by evaporation at the applied high temperatures and passed to the positive collector. The emitter and collector are connected through an external load through which the electrons flow back from the collector to the emitter. The flow of electrons constitutes a current providing useful electrical work in the external load. The magnitude of the output current through the load depends, generally, on the temperatures of the cathode and anode, the properties of the emissive materials used, and the spacing between the cathode and the anode.

The efficiency of a thermionic converter is affected by the space charge built up between the cathode and anode which tends to limit current flow. As a fundamental device to reduce the build-up of space charge, the spacing between the electrodes is kept low. Also, it has been the practiceto introduce cesium vapor into the converter and to control its vapor pressure by holding the temperature of the cesium reservoir temperature at a particular value, generally in the vicinity of 600 K. Cesium vapor also serves not only to decrease the space charge effect but also forms an adsorbed layer on the collector and emitter which reduces the work function of these electrodes. Thus, the rate of emission of electrons is effectively increased. Despite these expedients, thetheoretical efficiency of thermal converters has not been achieved in practice. Furthermore, the utilization of thermionic converters in practical systems has been hampered because of the large temperature differencethat must be maintained between the converter elements and the cesium reservoir. One reason for the failure to achieve theoretical efficiency is that a large fraction of available electrons at the emitter does not reach the collector because of scattering by cesium atoms. it has been shown that this loss of current increases as a function of pressure of the cesium. The cesium pressure, however, is selected, not with scattering in mind, but to provide a predetermined optimum emitter work function. It has been appreciated previously that it would be desirable to alter the characteristics of the emitter surface so that the desired work function could be obtained at a cesium pressure which is lower and causes less scattering. Also, it is known that oxygen can be used to cause a substantial increase in the bare work function and decrease the cesiated work function. Hence, the introduction into a converter of small quantities of oxygen would thus considerably improve the efficiency of operation.

Thus it is the primary objective of this invention to improve the output power and efficiency of thermionic converters.

Another objective of this invention is to simplify the structure of thermionic converters for use in practical systems.

These and other objectives of this invention are accomplished principally by placing a single compound containing an electropositive and an electronegative adsorbate in an appropriate position in the converter, the location depending on the characteristics of the particular compound and the design of the converter. At operating temperatures, the compound will provide the desired quantities of the adsorbates on the electrode surfaces.

Various requirements must be met in order for the electronegative and electropositive adsorbate providing compound to function successfully in a thermionic converter. The compound or its reaction products must not deleteriously attach the emitter or any other component of the converter or deleteriously affect work function characteristics. The compound must provide the desired quantities of oxygen and cesium at a compound temperature which is attractive for practical applications. lt is desirable that this temperature be close to that of the collector temperature so that the compound can be located on or adjacent to the collector. Furthermore, the compound must provide the desired quantities of the adsorbates reproducibly and stably over a long period of time. The compound can be of the type that will dissociate on location to give of? the adsorbates. Alternatively, it can be of the type that simply vaporizes upon heating, the vapor partially decomposing only when it comes into contact with the extremely hot emitter. An example of such compounds is the class of cesium oxides.

The single FIGURE of the attached drawing illustrates apparatus for providing suitable oxides as well as the converter utilizing those oxides.

The converter proper includes an emitter l l closely spaced and electrically isolated from a collector 12 by means of a ceramic ring 13. The collector 13 has a downwardly depending flange about which a heating coil 14 is disposed. At a lower point about an extension of the collector 13 a water jacket having channels for the flow of coolant is disposed. By suitable adjustment of heater input power and coolant flow the temperature of the collector may be controlled.

A annular opening is formed in the face of the collector and this opening communicates by means of radial passages with a central bore up through the center of the collector. A tube connects the bore with an additive reservoir 23. Temperature control of the reservoir 23is achieved by means of a heating coil 24 and a cooling strap 25. The foregoing elements, in addition to a vacuum enclosure 27 constitute the major elements of the converter of the present invention.

However, in the interests of clarifying understanding of the present invention, additional apparatus is illustrated. A vacuum and gas system is connected to the tube 22 which passes through a chill block 19, the temperature of which is controlled by means of a heater coil 20 and a coolant tubes 21 wrapped about the block. The tube 22 extends to a cesium reservoir'l6 which includes a relatively heavy sink at its lower end, the sink being controlled in temperature by means of a heater l7 and a cooling strap 18. From the reservoir 16, a tube 28 extends to provide communication between the interior of the converter and the reservoir 16. Thermocouples 26 in the emitter, collector, cesium reservoir and chill block permit monitoring of temperatures in the various elements.

In setting up the converter for operation, cesium is distilled from the cesium reservoir 16 to condense in the additive reservoir 23 by conventional adjustment of temperatures. Then, oxygen is introduced through the tube 22 to form a compound or solution with the cesium in the additive reservoir 23. The system is then reversed and all unreacted cesium and oxygen are pumped out through the tube 22, following which the tube 28 is pinched off to leave the converter with a single reservoir 23 containing cesium oxides.

The converter is operated conventionally with suitable electrical connections being made to the emitter and collector. Test data indicates that the volt-amp characteristics can be varied by changing the cesium-oxide temperature in the same fashion as they are varied by changingtemperatures of the cesium alone as is done conventionally. However, it has been found that, with the same emitter temperature, a cesium oxide reservoir operates from to 400 K. higher than the ordinary reservoir containing only cesium, thus approaching a value comparable to that cited above for typical collector operation. Accordingly, the amount of cooling required for the additive reservoir is vastly reduced, even though the reservoir is closely attached to the converter. The electrical data also indicated that cesium pressures are sufficiently less in the converter of the invention than in known devices to substantially reduce electron scattering and improve efficiency of operation.

Although only a single embodiment of the invention has been shown and described, it is believed that the concepts of utilizing a single reservoir and a single compound to provide both electropositive and electronegative adsorbates in a thermionic converter are novel. Suitable materials other than cesium may also be used without departure from the purview of the present invention which should be limited only by the spirit and scope of the appended claims.

What is claimed is:

1. In a thermionic converter having an emitter and a collector, apparatus for providing electropositive and electronegative substances between said emitter and said collector comprising a single reservoir containing a single compound one element of which is oxygen disposed adjacent said converter and means connecting the interior of said reservoir with the space between said emitter and said collector.

2. In a thermionic converter as defined in claim 1, the further combination therewith of means disposed about said reservoir for controlling the temperature thereof.

3. In a thermionic converter as defined in claim I, the combination wherein said single compound comprises cesium oxide.

4. In a thermionic converter as defined in claim 1, the combination wherein said electropositive substance is cesium and said electronegative substance is oxygen. 

1. In a thermionic converter having an emitter and a collector, apparatus for providing electropositive and electronegative substances between said emitter and said collector comprising a single reservoir containing a single compound one element of which is oxygen disposed adjacent said converter and means connecting the interior of said reservoir with the space between said emitter and said collector.
 2. In a thermionic converter as defined in claim 1, the further combination therewith of means disposed about said reservoir for controlling the temperature thereof.
 3. In a thermionic converter as defined in claim 1, the combination wherein said single compound comprises cesium oxide.
 4. In a thermionic converter as defined in claim 1, the combination wherein said electropositive substance is cesium and said electronegative substance is oxygen. 